Is the only way of setting the 'level' attitude of these brushless gimbals through performing the IMU calibration? Is there any way of adding a 'sub-trim' feature so if the calibration was very slightly out you can just add a few clicks of sub-trim to make the gimbal level?

I'm also noticing the ubiquitous yaw / roll coupling problem on the bench (rotate the camera in yaw and it also rolls slightly which will mess up the horizon) ... is this purely down to inaccurate IMU calibration? If so, presumably it's worth using a spirit level to make sure the surface that's used for calibrating is exactly flat in all dimensions?

You can use the RC initial angles for fine tuning the camera level. But better first to try to calibrate more accurately and attaching the IMU accurately. Yaw accurasy is most important on the IMU installation, if it is not correct, there will be unwanted interconnection between the axis. Pitch and roll errors can be corrected with the mentioned Init angles.

I am using Digital Pitch Gauge 2 by RCLogger which measures level in two perpendicular planes at the same time with 0.1deg accuracy. I put it in place of the camera, turn the motors off and fix the camera mounting plate in perfectly level position in both pitch and roll planes. It can be a bit tricky but I deviced a way to do it on my hexacopter quite easily and with high level of accuracy. Than I perform the IMU calibration of both sensors, camera and frame. In this way the camera is perfectly level on power-up in both planes and the gimbal holds horizon well too.

No. Never did the 6 axis, as it is as you say pain in you know what. It is tricky to attach the IMU in perfect allignment with the roll and tilt axis of the gimbal to avoid interaction. Once in place I do not want to do it every time I see the horizon drifts a bit and the IMU needs to be re-calibrated. With my method it is fast and effective way to bring the horizon back to level.

So you're saying you take your calibration rig with you when you go flying and if you see the horizon starting to drift / move with the yaw axis for example, you land and re-calibrate using your leveling jig and the digital pitch gauge?

No no no. I do not need to do it very often. As I mostly take stills the minor imperfection in horizon can be fixed in post processing. But if I see that the horizom becomes consistently off in one direction I simply recalibrate when I get home. Maybe once in a month or so.

Adding to the previous. If the gimbal is properly built and all 3 axis are truly perpendicular to each other there should be no interaction between axis movement. To put it simply, if you only move yaw for example the tilt and roll should not move. Or if you only tilt, roll and yaw should not move. These undesirable interactions are mostly related to the mechanics of the gimbal and accuracy of IMU placement.